Whether you’re using a desktop computer or a laptop, your computer’s motherboard contains an integrated battery. Unlike a standard laptop battery, the motherboard’s battery doesn’t power your computer while you’re using it. Quite the opposite, actually – the battery is tiny and only active when you’re not actually using your computer.

The motherboard’s battery is used for low-level system functions like powering the real-time clock and storing a computer’s BIOS settings. On newer computers, the battery may only be used for the clock.

What’s a BIOS?

Every computer has a Basic Input/Output System, known as a BIOS. (Newer computers actually have UEFI firmware. UEFI replaces the traditional BIOS, but largely serves the same role as the BIOS.) The BIOS is stored in a chip on your computer’s motherboard. When your computer boots up, the BIOS starts up, performs a power-on self-test (POST), and initializes the computer’s hardware. The BIOS then passes control over to a boot loader located on a device – usually your hard drive, but a boot loader can also boot off a USB device or optical disc. The boot loader then loads your operating system – Windows, Linux, Mac OS X, or whatever else is installed on your computer.

What’s a CMOS?

CMOS stands for complementary metal-oxide-semiconductor. Traditionally, BIOS settings were stored in CMOS RAM, which was powered by a battery when the computer was powered off. The battery was necessary because the settings would be lost if all power was lost, just as the data in your computer’s RAM is lost when it loses power.

Modern computers often don’t use CMOS RAM anymore. They store the BIOS settings in non-volatile memory, which means that the settings don’t need constant power to be saved.

Why Your Motherboard Needs a Battery

So if many computers now store BIOS settings in non-volatile memory that doesn’t need a battery, why do motherboards still come with batteries? Simple: Motherboards still include a Real Time Clock (RTC). This clock runs all the time, whether the computer is powered on or not. The real time clock is essentially a quartz watch, like the ones that people wear on their wrists (or used to wear on their wrists before cell phones came along.)

When the computer is off, the battery provides power for the real time clock to run. This is how your computer always knows the correct time when you power it on.

When the Battery Fails

As we all know from experience, batteries don’t last forever. Eventually, a CMOS battery will stop working. This may happen anywhere between two and ten years from when the computer (or its motherboard) was manufactured. If your computer is powered-on all the time, its battery will last much longer. If the computer is powered-off most of the time, its battery will die sooner – it’s using the battery more, after all.

If the battery fails on an older computer that stores its BIOS settings in CMOS, you’ll see error messages like “CMOS Battery Failure”, “CMOS Read Error”, or “CMOS Checksum Error” when you start the computer. You may also see more cryptic error messages, like “New CPU Installed” – the motherboard can’t remember that the CPU was installed previously, so it thinks it’s new every time you boot your computer.

Replacing the Battery

In these cases, you’ll need to replace the CMOS battery. On most computers, it’s a small, silver disc located on the motherboard. The exact type of battery is usually a CR2032 battery – also used in calculators, watches, and other small electronic devices.

“Pulling the CMOS battery” – in other words, removing it and re-inserting it – may also be used as a troubleshooting step on computers that store their BIOS settings in CMOS RAM. For example, if a computer has a BIOS password4 Creative Ways To Securely Password Protect Your Computer [Windows]4 Creative Ways To Securely Password Protect Your Computer [Windows]Passwords are a first line defense to your privacy and often they are the only one. If you are concerned about your data, you will want this barrier to be high and strong. As has...Read More, removing the CMOS battery and reinserting it will cause the computer to forget the BIOS password and all its other settings. (If the computer stores its password in non-volatile memory, this won’t help – although there’s probably a way to reset it using a jumper on the motherboard.)

You can also reset the BIOS settings from within the BIOS settings screen, assuming the computer is booting properly and you’re not locked out of the BIOS setup screen with a password. This option may be named “Clear CMOS” or “Reset to Defaults.”

Your email address will not be published. Required fields are marked *

Comment

Name *

Email *

Mike Walsh

September 22, 2017 at 7:19 pm

I don't really agree with your statement that soldered-on batteries necessitate a motherboard change, or a repair performed by the manufacturer.

I've got a 15-yr old Dell laptop; an original Inspiron 1100, from 2002. The CMOS battery was actually rechargeable, although soldered to the underside of the motherboard. It only expired last year, believe it or not; I sourced a replacement battery from a specialist battery supplier in London, in the UK; performed a complete strip down (easier on laptops of that vintage; you may recall they were a lot more spacious inside..!); removed the old battery, soldered the new one on, and re-assembled it.

I always see these pictures of someone holding a "new" uninstalled battery. Has anyone actually tried to pry it out of that snap connector? I tried, its almost impossible. So far I have a one in two success rate, the other attempt broke the motherboard. It would be nice to see an article on how to actually get the battery out of the holder.

My Asus Rampage II Extreme is not remembering the BIOS settings I have saved. It resets everytime even though I have replaced with new batteries and still no luck. Any possible fix? Thanks in advance =)

Had a case recently where a computer wouldn't boot. Went to manufacturer screen with options and then zip! Suspected power or hard drive - hard drive fine, pulled CMOS battery and replaced problem solved. Lots of the tech sites were talking about dead motherboard and other heavy stuff! (PC is HP DC7600) HAve had a few minor faults with this all of which people have said would be fatal. Usually fixed fairly cheaply :)

I've used the troubleshooting technique you mentioned earlier. - “Pulling the CMOS battery” – in other words, removing it and re-inserting it – may also be used as a troubleshooting step..."

It actually worked in resetting the BIOS settings.

My motherboard battery took 8 years to run out off juice. However, until now, I still don't bother changing it. It won't cause significant problems to your OS anyways. It is quite annoying to constantly see the error messages each time I cold boot though.

I think it should be ROM, not RAM.. ROM (Read Onlu Memory) and RAM (Random Access Memory) are "quite" different. We could only write data in ROM one time only, But in RAM we could store, wirte and read data manytime.. You should correct it.. But thanks for the info anyway.. :)

Well, the BIOS itself is stored in ROM, and the settings are stored in RAM.

Doc

February 20, 2013 at 10:27 pm

"CMOS stands for complimentary metal-oxide-semiconductor." Actually, it's "complementary." "Complimentary" means giving out a compliment, praise, or approval; "complementary" means it complements, or completes, something. The two words have different meanings, but are often confused for one another.